Set detumbled_email address before saving emails

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/work_items/827

This MR adds a before_save hook to the Email model that sets the detumbled_email attribute. This attribute will be used to identify duplicate email addresses when alternative addressing schemes are used. Gmail addresses are normalized by removing the . character and everything between the + and the @ character in an email address. For example, test.user+gitlab@gmail.com will become testuser@gmail.com when normalized. Non-Gmail addresses only strip the characters between the + and @ characters, so test.user+gitlab@example.com will be normalized to test.user@example.com.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enable(:store_detumbled_email)

2. Create a user with an alternative gmail address and confirm the user. The detumbled_email attribute should be present in the email record.

   admin = User.first
   user = Users::CreateService.new(admin, { name: 'new_user123', username: "new_user123", email: "new.user123+gitlab@gmail.com", password: "asdfasdf123"}).execute 
   user.confirm
   user.emails.first.detumbled_email
   => "newuser123@gmail.com"

3. Create another user with an alternative email address from a non-gmail domain. The detumbled_email should still contain the . characters.

   user = Users::CreateService.new(admin, { name: 'new_user456', username: "new_user456", email: "new.user456+gitlab@example.com", password: "asdfasdf123"}).execute 
   user.confirm
   user.emails.first.detumbled_email
   => "new.user456@example.com"